Industrial boilers, such as oil-fired, coal-fired and trash-fired boilers in power plants used for electricity generation and waste incineration, as well as boilers used in paper manufacturing, oil refining, steel and aluminum smelting and other industrial enterprises, are huge structures that generate tons of ash while operating at very high combustion temperatures. These boilers are generally characterized by an enormous open furnace in a lower section of the boiler housed within walls constructed from heat exchanger tubes that carry pressurized water, which is heated by the furnace. An ash collection and disposal section is typically located below the furnace, which collects and removes the ash for disposal, typically using a hopper to collect the ash and a conveyor or rail car to transport it away for disposal. In case of pulp and paper black liquor recovery boilers, the products of the combustion in the furnace are directed to a green liquor tank to recover the inorganic cooking chemicals used in the pulping process.
A superheater section is typically located directly above the furnace, which includes a number of panels, also called platens or pendants, constructed from heat exchanger tubes that hang from the boiler roof, suspended above the combustion zone within the furnace. The superheater platens typically contain superheated steam that is heated by the furnace gas before the steam is transported to steam-driven equipment located outside the boiler, such as steam turbines or wood pulp cookers. The superheater is exposed to very high temperatures in the boiler, such as about 2800 degrees Fahrenheit [about 1500 degrees Celsius], because it is positioned directly above the combustion zone for the purpose of exchanging the heat generated by the furnace into the steam carried by the platens. The boiler also includes a number of other heat exchangers that are not located directly above the furnace, and for this reason operate at lower temperatures, such as about 1000-1500 degrees Fahrenheit [about 500-750 degrees Celsius]. These boiler sections may be referred to as a convection zone typically including one or more pre-heaters, re-heaters, superheaters, and economizers.
There is a high demand for thermal energy produced by these large industrial boilers, and they exhibit a high cost associated with shutting down and subsequently bringing the boilers back up to operating temperatures. For these reasons, the boilers preferably run continuously for long periods of time, such as months, between shut down periods. This means that large amounts of ash, which is continuously generated by the boiler, must be removed while the boiler remains in operation. Further, fly ash tends to adhere and solidify into slag that accumulates on high-temperature interior boiler structures, including the furnace walls, the superheater platens, and the other heat exchangers of the boiler. If the slag is not effectively removed while the boiler remains in operation, it can accumulate to such an extent that it significantly reduces the heat transfer capability of the boiler, which reduces the thermal output and economic value of the boiler. In addition, large unchecked accumulations of slag can cause huge chunks of slag to break loose, particularly from the platens, which fall through the boiler and can cause catastrophic damage and failure of the boiler.
The slag accumulation problem in many conventional boilers has been exacerbated in recent years by increasingly stringent air quality standards, which have mandated a change to coal with a lower sulphur content. This low-sulphur coal has a higher ash content and produces more tenacious slag deposits that accumulate more quickly and are more difficult to remove, particularly from the superheater platens. To combat this problem, the industry has developed increasingly sophisticated boiler cleaning equipment that operates continually while the boiler remains in operation. In particular, water cannons can be periodically used to clean the boiler walls in the open furnace section, and steam, water, air, and multi-media sootblowers can be used to clean the heat exchangers. These sootblowers generally include lance tubes that are inserted into the boiler adjacent to the heat exchangers and operate like large pressure washers to clean the heat exchangers with steam, water, air or multi-media blasts while the boiler remains in operation.
Fireside deposit accumulation in both power and recovery boilers not only reduces the boiler thermal efficiency, but can also lead to costly unscheduled shutdown due to the plugging of the gas passages. Although full plugging of the gas passages in power boilers can be considered a rare case, localized plugging can significantly accelerate the gas velocity and increase the risk of tube erosion.
Generally, sootblowers are configured with balanced jets to minimize the torque imposed on the sootblower lance. A first type of conventional sootblower has perpendicular nozzles with jets directed at opposing right angles to the major axis of the sootblower. Sootblowers with perpendicular nozzles work well at removing thin slag deposits and deposits inset from the leading edges of the platens but are less effective at removing thick slag deposits on the leading edges. An alternative type of conventional sootblower has lead-lag nozzles with jets directed at opposing acute angles to the major axis of the sootblower. Sootblowers with lead-lag nozzles work well at removing thick deposits on the leading edges of the platens but are less effective at removing thin deposits and slag deposits inset from the leading edges. At present, there is a need for a sootblower that successfully removes thick slag deposits on the leading edges of the platens, thin deposits on the leading edges, as well as slag deposits inset from the leading edges of the platens.